I had my tranny on my 91 420 fail in the same way. However I pulled over right away and had it towed. It failed about 1 month after changing the transmission fluid. The shaking you describe happened to me also. It turned out to be internal hard parts of the transmission rubbing each other. In doing so the transmission is working against itself and generating a lot of heat.

Where was the antifreeze comming from? You say there is antifreeze "in the pan". What pan are you talking about?

Often times the transmission cooling lines in the radiator fail and antifreeze is sucked into the transmission fluid cooling circuit. If that happened you tranny is toast and a total rebuild is needed. If that is indeed what happened, I can see how anitfreeze and ATF might leak out of the top of the transmission at the case vent.